1. Field of the Invention
The present invention relates to a digital watermarking technology, and it particularly relates to a method for embedding a digital watermark and an encoding apparatus and a decoding apparatus using the same method.
2. Description of the Related Art
The number of Internet users has rapidly increased in recent years and we are now entering the age of the broadband, or a new stage in the utilization of the Internet.
Since communication bandwidth has greatly expanded in broadband communication, the distribution of items containing large bodies of data such as audio, still image, and video can be enjoyed with ease. When the distribution of such digital items becomes popular, a highly efficient method of protecting the copyright of their contents will be required.
In the present situation, the copyright is not protected well so that users can easily copy such contents distributed via the Internet. Therefore, technology for embedding information on the originator of the content and the user into the content as a digital watermark has been developed. By using this watermarking technology, it becomes possible to extract the digital watermark from the content distributed via the network, and thereby detect an illegal use and track the distribution route of an illegal copy.
A digital watermark is embedded in the content data in a way that the watermark is perceptually invisible to users in order to prevent malicious attacks. However, the content data will be subject to various operations during the distribution and utilization, for instance, signal processing such as compression, various types of filtering, modification by users, or attacks on the watermark information. In such processes, a part of the digital watermark may be possibly altered or removed. Therefore the digital watermark should be robust enough to withstand such operations.
Various digital watermarking techniques have been developed in order to improve the robustness of digital watermarks. For instance, Japanese Patent Application Laid-Open 2000-13587 discloses a method for embedding a digital watermark with a high level of robustness while providing a degree of freedom for the process of embedding the digital watermark information. Such a digital watermarking technique adopts a method for embedding a digital watermark in high frequency components such as an edge or a region with a significant change in an image texture, according to the characteristics of the human visual system, and therefore the technique depends to a large degree on the individual content data and has limitations on the generality and flexibility so as to be difficult to improve the robustness of the embedded watermark against various operations which might be carried out on the contents data.